Customers can even upgrade their SIMs for free on Vodafone's official website.

For those prepaid customers using 4G handsets but still have an outdated SIM card, Vodafone India is offering them 4GB data on its 4G network on upgrading.

The telecom operator has been sending multiple messages to its subscribers with old 3G SIM cards operating in 4G handsets. “Your SIM will expire shortly. Please visit your nearest Vodafone retailer to upgrade to a 4G SIM soon,” the message read.

Vodafone has made new 4G SIM cards available with retailers and kirana shops where customers can go to upgrade their existing SIM cards.

The procedure to upgrade a SIM card is as follows:

From customers’ existing Vodafone number, an SMS will have to be sent to 55199 in the prescribed format: 'SIMEX <your 19-20 digit new 4G SIM card number>' (Available on the new SIM card/jacket given to you by the retailer).

The SMS to 55199 is toll free.

Customers will then receive a response SMS from 55199, “Hi! We have received your request for SIM upgrade starting with digits (19-20 digit SIM number). Pls reply with the last 6 digits of new SIM number immediately.”

Last six digits of the new SIM number will need to be sent to 55199 immediately after receiving the SMS mentioned above.

Post this, the company will initiate a two hour procedure post and a second confirmation message will be sent wherein the customer can still ‘cancel’ the upgrade by replying with ‘CAN’.

Subsequent messages will revolve around time left for the procedure to be completed and customers will have to switch to the new 4G SIM cards once the network on the old SIM card goes completely.

Post the activation of the new SIM card Vodafone will offer 4GB of 4G internet for 10 days.

New SIM packet instructions read, "Total maximum value of Rs 10 towards activation of service (inclusive of taxes). No charges towards SIM." Customers can even upgrade their SIMs for free on Vodafone's official website.

Customers facing any issues with the SIM exchange process can call 199, toll free number, or seek assistance from Vodafone staff at any of its retail stores.

However, customers must ensure their smartphone devices are 4G enabled before switching to the new 4G SIM cards.